This invention relates in general to non-destructive techniques for measuring surface parameters of samples and, in particular, to a system for measuring polarimetric spectrum and other properties of a sample.
For a number of industrial applications, it is useful to determine sample surface parameters such as thicknesses of thin films and their refractive indices. These parameters may be determined by a number of techniques. Among the better known non-destructive testing techniques are those of spectroreflectometry and spectroscopic ellipsometry.
In spectroreflectometry, an incident radiation beam reflects from a sample, and the intensity of the reflected radiation is analyzed to determine properties of the sample. The incident radiation may include multiple frequency components so that a spectrum of measured data (known as the reflectance spectrum or relative reflectance spectrum) is measured. U.S. Pat. No. 5,747,813 describes a type of broadband spectroreflectometer; this patent is incorporated herein in its entirety by reference.
A spectroscopic ellipsometer is described in U.S. Pat. No. 5,608,526, which is incorporated herein in its entirety by reference. As described in this patent, an incident radiation beam having a known polarization state is reflected from a sample, generally at a large angle of incidence and the polarization of the reflected radiation is analyzed to determine properties of the sample. The polarization of the reflected radiation is altered from that of the incident beam so that the change of polarization state in amplitude and phase is measured to determine the thicknesses and refractive indices of thin films on the surface of the sample.
Spectroreflectometry is simpler and less expensive than spectroscopic ellipsometry, but spectroscopic ellipsometry is more accurate and sensitive to surface properties than spectroreflectometry. While spectroreflectometry and spectroscopic ellipsometry are useful non-destructive testing techniques, it may be difficult to use such techniques to measure certain surface parameters, such as surface bireflingence.
It is therefore desirable to provide an improved measurement system for measuring sample parameters such as birefringence. It may also be desirable to provide a polarimeter system for measuring properties of the sample surface to supplement the above-described non-destructive testing techniques.
The polarimetry system proposed by Applicants retains the simplicity of the spectroreflectometer but is as sensitive to surface properties as the spectroscopic ellipsometer. A polarized sample beam of broadband radiation is focused onto the surface of a sample and the radiation modified by the sample is collected. The sample beam focused to the sample has a multitude of polarization states. The radiation modified by the sample is analyzed with respect to a polarization plane to provide a polarimetric spectrum. Thickness and refractive index information may then be derived from the spectrum. Preferably the polarization of the sample beam is altered only by the focusing and by the sample, and the analyzing is done with respect to a fixed polarization plane.
In the preferred embodiment, the focusing of the sample beam and the collection of the modified radiation are repeated employing two different apertures to detect the presence or absence of a birefringence axis in the sample. In another preferred embodiment, the above-described technique may be combined with ellipsometry for determining the thicknesses and refractive indices of thin films.